Casting mould for casting a cast part and use of such a casting mould

ABSTRACT

The present invention relates to a casting mould for casting a casting, in particular an engine block for a combustion engine, as well as to the use of such a casting mould, comprising mould parts  1, 2, 3  and casting cores  4, 5, 6, 7  produced from moulding sand, and at least one casting core  4, 5, 6, 7  for the formation of a space in the casting. The casting mould according to the invention makes it possible, in a simple manner, for castings to be produced in which at least one locally closely confined section is formed with a microstructure which differs from the remainder of the casting. This is achieved in that a section of the casting core  4, 5, 6, 7  is formed by a chill  9  which is manufactured from a material of which the thermal conductivity is many times higher than the thermal conductivity of the other section  8  of the casting core  4, 5, 6, 7,  consisting of moulding sand.

The invention relates to a casting mould for casting a casting,comprising mould parts produced from moulding sand and casting cores andhaving at least one casting core for the formation of a space in thecasting.

Casting moulds of this type are used in particular for the casting ofengine blocks for combustion engines. In this context, the casting corein the engine block casting forms the individual combustion chamber,while the other mould part forms the head surface of the engine block,on which, during assembly, the cylinder head of the individualcombustion engine is placed and secured, as necessary, as an add-onpart.

In practical operation, it is precisely in the area between the spaceformed in the casting by the casting core and the surface on which theadd-on component is mounted that substantial loads may be incurred bythe effect of heat or mechanical loading. The stresses resulting fromthis in the casting material surrounding the space in the area affectedcan be so great that crack formation and fractures may occur. Thisproblem is particularly critical with engine blocks for moderncombustion engines which are cast from light metal or light metalalloys, such as aluminum or an aluminum casting alloy. Because of theever increasing demands being made on the one hand on the performanceand, on the other, on the minimization of the weight of such engines, itis specifically in the area of the opening, allocated to the cylinderhead, of the cylinder chamber or combustion chamber respectively formedthat extreme loads are incurred which can lead to local material failurein the area of the inner surfaces of the combustion chamber. With amulti-cylinder engine block, with combustion chambers arranged closelyadjacent to one another, this damage affects in particular the webs bymeans of which the cylinders are separated from one another on thecylinder head side. With modern engine designs, it is precisely in thisarea that the accumulation of casting material is reduced to a minimum,in order, by arranging the combustion chambers as close to one anotheras possible, to attain with optimum energy utilization the shortestpossible structural length of the engine block.

Attempts have been made to improve the mechanical and thermal stressresistance of the casting material in the area of the sections ofcastings which in practical operation are particularly subject to stressby the sections affected being specifically cooled in the course ofsolidification, such that they form at that point a microstructure whichis favorable in respect of the particular demands imposed.

For this purpose, for example, according to the method described in DE195 33 529 C2 for the casting of an engine block made of aluminum in asand mould, the cylinder cavities of the engine block are formed by diesinserted into the sand mould, these consisting of a brass material. Thealuminum solidifies on the surface of the brass die as a consequence ofthe higher thermal conductivity of the metal of the die more rapidlythan on the surface of the sand mould. As a result, a microstructure isproduced there to an adequate depth, such that it possesses a higherresistance to the loads incurred in practical operation on the runningsurfaces of the cylinders.

Practical experience with the known method described above has shownthat it is necessary for the brass dies to be pre-heated before the meltis poured into the individual casting mould. Otherwise the risk arisesthat the casting metal coming in contact with the metal of the die willsolidify too rapidly and cracks or metal fractures will appear. The needfor pre-heating the dies not only leads to higher manufacturing effortand expenditure, but also brings with it the problem that it is onlypossible with great difficulty to create, in specific closely confinedareas of the combustion or cylinder chambers of combustion engines, aspecific microstructure which will resist the conditions which prevailin practical operation.

Against this background, the object of the invention was to provide acasting mould which makes it possible, in a simple manner, for castingsto be produced in which at least one locally closely confined section isformed with a microstructure which differs from the remainder of thecasting. In addition to this, it should also specify an advantageous useof such a casting mould.

With regard to the casting mould, this object is resolved according tothe invention by the subject matter of Claim 1. Advantageous embodimentsof this solution are provided in the claims referring back to Claim 1.

With regard to the use, the object referred to above is resolvedaccording to the invention by the subject matter of Claim 11.Advantageous embodiments of this use are specified in the claimsreferring back to Claim 11.

With a casting mould according to the invention, the casting coreforming the space in the casting in each case is divided into at leasttwo sections. The first section in this situation is formed by a chill,while the other section usually consists of mould sand placed in thearea of the sand casting. Such mould sand is formed in a known mannerfrom mixtures of basic mould material, as a rule free-flowing, and abinder. This mould sand mixture is shaped in a core mould to theindividual mould part or casting core and then secured by a suitablemechanical, chemical, and/or thermal treatment in such a way that thepart or core obtained has adequate shape stability for the castingprocess.

The chill used according to the invention as a part of the casting coreforming the space, has according to the invention a thermal conductivitymany times higher than the mould sand from which the other section ofthe casting core is produced. Accordingly, in the area of the castingmould in which the melt poured into the mould comes in contact with thedie locally delimited accelerated cooling takes place. By the selectionof the material and the volume of the chill it is possible, in thissituation, for the speed at which the locally delimited cooling takesplace to be directly influenced, as well as the quantity of heat drawnoff the casting metal in each case.

In this situation, by shaping the casting chill, the extent of the areaover which the specifically accelerated drawing off of heat should takeplace can be adjusted in a simple manner by the overall design of thechill. If it is intended, for example, when casting an engine block,that the casting material surrounding the cylinder space to be formed inthe engine block should be specifically and massively cooled for thepurpose of forming a particularly stress-resistant castingmicrostructure over a specific part length of the cylinder space, thenfor this purpose the casting core forming the individual cylinder spaceis provided with a ring-shaped chill, the height of which corresponds tothe specifically cooled part length of the cylinder chamber, takingaccount of the heat migration.

As well as the possibility created by the invention of carrying outlocally closely confined specific cooling in the area of the innersurfaces of a space to be formed in a casting, a further advantage ofthe invention which is of importance in actual practice lies in the factthat the chill used according to the invention does not need to besubjected before its use to either a special surface treatment or bepre-heated. It has been shown, for example, that with a suitable choiceof material and shaping, the chill can be removed in a simple mannerfrom the finished solidified casting. In addition to this, a chill usedaccording to the invention produces, in the area of the casting materialcoming in contact with it, such a good surface quality that the chillcan be inserted into the casting mould without a coating. Accordingly,with a casting mould according to the invention, the casting materialcomes in direct contact with the chill, such that a particularly rapidheat removal takes place, not impeded by any intermediate layerfunctioning as an insulator.

As a result, the invention in this way makes the economical manufactureof castings possible in which, in the area of a space to be formed inthe castings, locally closely confined zones are present with a castingmicrostructure created by accelerated cooling.

The properties achieved by the embodiment according to the inventionmake casting moulds according to the invention particularly well-suitedfor the casting in large-scale series of engine blocks for combustionengines, wherein their advantages come to the fore in particular in thecasting of such engine components made of light metal or light metalalloys, in particular of aluminum or aluminum alloys. It is particularlywith the casting of engine blocks that the possibility created by theinvention, of specific and massive cooling in a closely confined partarea of the individual cylinder chamber has a particularly favorableeffect. This becomes particularly noticeable if the zone of the engineblock which is to be specifically cooled is the area at which theindividual cylinder chamber merges into the assembly surface on whichthe cylinder head of the engine is mounted. With the aid of theinvention, it is possible at that point specifically for the castingmaterial to be cooled in a specific manner in the course of itssolidification such that material properties which are always in keepingwith the requirements arising in practical operation are present tooptimum effect.

One variant of the invention which is particularly advantageous withregard to mass production makes provision for the chill to bemanufactured from cast iron Chills consisting of cast iron can beeconomically manufactured and have a thermal conductivity which isfavorable with regard to the processing aim being striven for in thissituation.

For the reasons already mentioned, a casting mould according to theinvention is particularly well-suited for the manufacture of suchcastings in which a microstructure is to be created in the area of atransition between the space to be created in the casting and anassembly surface, which is capable of supporting the loads occurringduring operation. One embodiment of the invention which is of importancein practice therefore makes provision for at least one of the mouldparts of a casting mould according to the invention to form an assemblysurface on the casting, on which an add-on part can be mounted on thecasting after its solidification, and for the mould core, in particularwith the chill, to border on this mould part. In order to implement thedesired locally delimited accelerated cooling in a reliable manner, inthis situation the thermal conductivity of the chill should likewise bemany times higher than the thermal conductivity of the mould partforming the assembly surface.

The arrangement of the chill and of the other section of the castingcore forming the space in the casting in the correct position can beassured by a simple procedure, in that elements are formed on the chilland the other section of the casting core, by means of which the chilland the other section of the casting core are connected to one anotherin positive fit.

If the chill is used in such a way that it cools a zone of the castingwhich is to be produced at the transition into the assembly surface, itis advantageous if at least one element is formed on the chill whichengages in positive fit into a correspondingly shaped cut-out apertureof the mould part.

In this situation, this element is preferably formed as a projectionengaging into the mould part. The chill which engages in this way intothe mould part concerned extends in this case over the assembly surfaceto be produced on the casting, such that the occurrence of burrs orcomparable mould defects in the area of the aperture of the individualcylinder chamber is reliably prevented. Inasmuch as the inner surface ofthe projection is inclined at an oblique angle in relation to thelongitudinal axis of the chill, in such a way that the diameter of theaperture of the casting core widens in the direction of the mould part,in this situation the removal of the chill from the finished casting canbe made additionally easier.

With the large-scale technical use of a casting mould according to theinvention in particular, it is advantageous if the casting core with thechill is held by a ram extending into the casting core. This ram can, onthe one hand, be used for the precisely positioned holding of thecasting core. In addition, such a ram also allows, when coupled to anappropriate adjustment device, automatic installing of the casting coreitself.

Due to the high wear loads which arise in the area of the runningsurfaces of the cylinder chambers of a combustion engine, it isfrequently necessary for what are referred to as “liners” to be castinto the engine block. These liners are, for example, prefabricatedtubular structural elements made of grey cast iron, of which the innerdiameter corresponds to the inner diameter of the cylinder chamber to beformed in the engine block and the inner surfaces of which form therunning surfaces in the finished engine block, along which, inoperation, the piston of the combustion engine moves. With a castingmould according to the invention, liners can also be cast into thecasting which is to be produced, with the simultaneous exploitation ofthe advantages of the invention, if the casting core carries on itsouter surface a liner which is to be cast into the casting.

The invention is described hereinafter in greater detail on the basis ofa drawing representing an embodiment.

The single FIGURE shows in diagrammatic form a casting mould G forcasting an engine block for a straight-4 combustion engine.

The casting mould 1 is composed of different mould parts 1, 2, 3 andcasting cores 4, 5, 6, 7 produced in each case from moulding sand, whichare formed in each case from a section 8 prefabricated from mouldingsand and a chill 9.

The mould part 1 represented at the top in the FIGURE is what isreferred to as a “bottom core”, which forms the assembly surface 10 onthe engine block which is to be cast, also referred to as the “top-decksurface”, on which, in the course of the assembly of the combustionengine a cylinder head is secured, not represented here. The mould part2, arranged opposite the mould part 1, represents what is referred to asthe “crank chamber core”, which forms the crank chamber in the engineblock to be cast.

The casting cores 4, 5, 6, 7 are in each case retained by a ram 11, 12,13, 14. The rams 11-14 are coupled to an actuator device, not shownhere, which moves them out of a mounting position, in which the castingcores 4, 5, 6, 7 are mounted on them, into the position represented inthe FIGURE. The front section 15 of the rams 11-14 projecting into theindividual casting core 4-7 is in this case formed such as to taperconically over the entire height of the casting cores 4-7 in thedirection of the free end of the ram, in order to be able to draw therams 11-14 unimpeded out of the casting cores 4-7 after the conclusionof the casting process.

The section 8 of the casting cores 4-7, made of moulding sand, has abeaker shape, wherein the interior which it surrounds is adapted to theshape of the front section 15 of the rams 11-14 in such a way that thesection 15 concerned is located in positive fit in the interior. In thissituation, a projection 16 is formed at the front free end of thesection 15, in each case arranged centrically on the face side of thecasting cores 47, which engages into a correspondingly shaped cut-outaperture 17 of the mould part 2 and is likewise held in this in positivefit.

On its opposite edge, facing the chill 9 which is produced as amonolithic block made of grey cast iron, the section 8 of the castingcores 4-7 has in each case a circumferential shoulder 18, into which aring-shaped circumferential projection 19 engages, formed at theface-side edge of the chill 9 facing the section 8. In this way, thechills 9 of the casting cores 4-7 are linked in positive fit to thesection 8, made of moulding sand, of the individual casting core 4-7.

The chills 9 in this situation themselves have a ring-shaped design. Theinterior surrounded by them is in this situation adapted to the shape ofthe front section 15 of the rams 11-14 facing them in each case, suchthat the chills 9 are likewise retained in positive fit and essentiallyfree of play on the rams 11-14 allocated to them.

On their upper edge, facing the shaped part 1, a circumferentialprojection 20 is likewise formed in each case on the chills 9, the outercircumferential surface of which, as with the projection 19, mergessmoothly into the circumferential surface 21 of the main section of theindividual chill 9.

The chills 9 in this situation have a slightly conical shape, taperingin the direction of the free end of their projection 19. For thispurpose, the common circumferential surface 21 of the chills 9 isinclined by an angle of at least 2° in relation to their longitudinalaxes L, congruent with the longitudinal axes of the rams 11-14. In thecorresponding manner, the inner surfaces of the chills 9 are alsoinclined. The conical outer shape of the chills 9 facilitates theremoval of the chills 9 from a liner 22, which is held in positive fitby the individual casting core 4-7 in each case, and, aftersolidification, remains in the engine block which is to be cast in thecasting mould G. In the engine block which is to be cast, the liners 22surround the cylinder chambers which are to be formed by the castingcores 4-7 in the engine block.

The height of the projections 20 of the chills 9 is dimensioned in sucha way that in each case, with the casting cores 4-7 and mould parts 1-3positioned ready in the casting mould G, they engage over the assemblysurface 10 into a correspondingly shaped cut-out aperture 23 of themould part 1. In this way, burr formation can be reliably prevented inthe area of the opening of the cylinder chambers of the engine blockwhich is to be cast.

In the casting mould G, in an inherently known manner, feeders andfilling channels are formed, not shown here, by means of which thealuminum melt poured into the casting mould G flows into the mouldcavity 24 surrounded by the casting mould G. Heat is drawn off in anaccelerated fashion, via the liners 22 and the individual chill 9, fromthe melt which in this situation passes into the areas 25 of the mouldcavity 24 adjacent to the chills 9, such that it solidifies more rapidlythan the melt present in the other zones of the mould cavity 24.Accordingly, in the areas 25 of the engine block which is to be cast, analigned finer-grain microstructure is formed, such that the section ofthe engine block adjacent to the assembly surface 10, which in this areais subjected to particular heavy loading, will reliably resist thethermal and mechanical loads occurring in practical operation.

1. Casting mould for casting a casting comprising mould parts produced from moulding sand and having at least one casting core for the formation of a space in the casting, wherein a section of the at least one casting core is formed by a chill which is manufactured from a material having a thermal conductivity that is many times higher than the thermal conductivity of another section of the at least one casting core, the another section consisting of casting sand.
 2. Casting mould according to claim 1, wherein the chill is manufactured from cast iron.
 3. Casting mould according to claim 1, wherein at least one mould part forms an assembly surface at the casting, at which an add-on part can be fitted to the casting after solidification, and in that the at least one casting core adjoins the at least one mould part.
 4. Casting mould according to claim 3, wherein the thermal conductivity of the chill is many times higher than the thermal conductivity of the at least one mould part forming the assembly surface.
 5. Casting mould according to claim 1, wherein at least one element is formed on the chill and the another section of the at least one casting core, by means of which the chill and the another section of the at least one casting core are connected to one another in positive fit.
 6. Casting mould according to claim 1, wherein at least one element is formed on the chill, which engages in positive fit into a correspondingly shaped cut-out aperture of at least one mould part.
 7. Casting mould according to claim 6, wherein the element is formed as a projection engaging in the at least one mould part.
 8. Casting mould according to claim 7, wherein the inner surface of the projection is arranged at an oblique angle in relation to the longitudinal axis of the chill.
 9. Casting mould according to claim 7, wherein the projection engages over an assembly surface into the at least one mould part.
 10. Casting mould according to claim 1, wherein the at least one casting core is held with the chill by a ram extending into the at least one casting core.
 11. Use of a casting mould formed according to claim 1, for the casting of an engine block for a combustion engine.
 12. Use according to claim 11, wherein use is made as a casting material of a light metal or a light metal alloy.
 13. Use according to claim 11, wherein the at least one casting core of the casting mould carries on an outer surface a liner, forming, in the finished casting, an inner wall of the space to be formed by the at least one casting core.
 14. Use according to claim 11, wherein the at least one casting core of the casting mould forms a cylinder chamber of the engine block.
 15. Use according to claim 11, wherein an add-on part is a cylinder head. 